This invention relates to a process for producing, having non-collapsing properties and a process for recovering iron components from the processed slag.
Slag produced from a steel making process which used LD converters or electric furnaces (hereafter referred as steel slag) usually takes a fairly long time to reach a stable state. For example, what might appear to be a hard lump often swells and crumbles, after a while, to finer particles. Flat surfaces formed of land fill of unweathered steel slag in the sea or valley would later exhibit undulating surfaces causing great difficulties to any buildings erected hastily after land reclamation. A road made using steel slag for the base as a substitute for crushed stone would almost for sure become unusable fairly soon.
Unless a fairly long time is allowed to complete the weathering of steel slag it can not be used as a substitute for crushed stone or sand for constructional work or as a reliable material for land fill. For weathering, a vast tract of land is of course necessary, which is especially costly in Japan. Recently dumping in the sea is regulated from the view point of pollution control and some steel mills are forced to haul steel slag long distance to designated areas in the ocean for dumping.
The weathering of steel slag is considered to be caused by volume increase of Free CaO and 2CaO.SiO.sub.2 (hereafter to be referred as C.sub.2 S), the former swells by absorbing water and changes to Ca(OH).sub.2 and the latter by a phase shift.
Namely, 2CaO.SiO.sub.2, which takes four forms or phases, .alpha..fwdarw..alpha.'.fwdarw..beta..fwdarw..gamma., rapidly and greatly increases the volume thereof when it transforms from its .beta. form to its .gamma. form. Due to the above sudden swelling, 2CaO.SiO.sub.2 collapses or dusts. One way to suppress the weathering of slag is a rapid cooling of steel slag, but this method is cumbersome and the practical effect does not seem to be too great.
We have found a process to prohibit the weathering of steel slag by adding red mud produced from the aluminum industry to molten steel slag, thereby transforming Free CaO and C.sub.2 S to other compositions which would will not swell or crumble. We have further found that the addition of reducing agents such as coke or blast furnace thickner dust along with red mud to molten steel slag has the effect of reducing some of the iron oxides contained mainly in molten slag to metallic form, thereby recovering the thus reduced metallic iron, together with the metallic iron left from the previous process, and some of the magnetic iron oxide.
Red mud is the residue of the process of treating bauxite by caustic soda under high pressure. Particle size of red mud is very small. Usually more than half of the total particles are under 2 microns. Sometimes the iron content of red mud is close to 50% (all "percent" in this specification are by weight.), but further up grade is considerably difficult. Fine particles of peculiar red color tend to cause pollution. Therefore red mud has heretofore generally been nothing but a cause for headache. We have, for more than 20 years, been engaged in steel slag treatment on the one hand and investigation of economic treatment of red mud on the other, and we have succeeded in combining these two fields of technology which have so far seemed discretely separate.
The melting point of red mud is about 1250.degree. C., which is 200.degree.-300.degree. C. lower than that of steel slag.
It is an object of the present invention to provide a process for producing slag having non-collapsing properties which is characterized in that red mud and molten slag produced by a converter furnace or an electric furnace both of which have been used heretofore but in extremely limited industrial areas but which can now be utilized to a full extent.
It is another object of the present invention to provide a process for producing slag having non-collapsing properties without necessitating quenching (rapid cooling) of the molten slag wherein the process is characterized in that red mud is charged into the molten slag produced by a converter furnace or an electric furnace so that the charged and melted red mud reacts with the molten slag resulting in the production of processed slag which has non-collapsing properties.